nir: Move shared_memory_explicit_layout bit into common shader_info
Move it out of the "cs" sub-struct, since the bit can be used for other shader stages in the future.
This also removes a subtle issue in spirv_to_nir: info.cs.shared_memory_explicit_layout was used without checking for the CS shader stage. It ended up being "harmless" since the effects also depended on presence of shared variables.
Fixes: 5de6c597 ("spirv: Implement SPV_KHR_workgroup_memory_explicit_layout")